1. Field of the Invention
The present invention relates to a method of semi-molten metal injection molding to mold thin products from a semi molten metal, and an apparatus for producing the same.
2. Prior Art
As a method of producing metal molded articles having higher internal quality than that made by die-casting method, semi-molten metal injection molding method has been known, in which molten metal (for example, magnesium alloy), in a semi-melting state held at a temperature of not more than the liquidus temperature of the alloy is injected into a cavity of a mold, as disclosed in Japanese Patent Publication JP-B 2-015620, corresponding to U.S. Pat. No. 4,694,882. Since the method of semi-molten metal injection molding method is possible to mold the melt at relatively low temperatures, it can have longer useful life of the mold than the die-casting method, and moreover, improve molding accuracy for products.
When thin molded products having thickness of not more than 1.5 mm in a product portion corresponding to a narrow cavity are formed by injection molding, the molten metal is solidified too rapidly in such a narrow cavity to mold the sound products. Thus, a higher injection speed is required for molding the melt without such a problem. The semi-molten metal injection molding is excellent without substantial burrs. For die casting, high-speed injection often provides much burrs, which are economically disadvantageous, and also causes disturbance in the molten metal flow, which leads to even lower internal quality.
In the method of semi-molten metal injection molding, however, since metal materials are molded in a semi-melting state at a temperature of not more than a liquidus temperature of the alloy, the fluidity of the molten metal tends to be lower to increase the possibility of the cavity not completely to be filled with the semi-molten metal. Thus, Without proper molding conditions set, it would be difficult to apply semi-molten metal injection molding method to molding thin, sound products having no defects.
The present invention has been accomplished in consideration of the problems described above. An object of the present invention is to provide a method of the semi-molten melt injection molding of thin, sound products by setting the proper molding conditions to maintain the fluidity of the melt at a sufficient level.
Another object of the invention is to provide molding conditions to be set, for the semi-molten melt injection molding of thin, sound products.
Further object of the invention is to provide an apparatus for semi-molten metal injection molding of thin, sound products, by setting the proper molding conditions to maintain the fluidity of the melt at a sufficient level.
According to the present invention, in order to achieve the object described above, prior to semi-molten metal injection, the size of crystal grains suspended in the semi-molten melt is defined as being at a sufficiently lower fixed level with regard to average thickness of a product to be molded, not to reduce fluidity of the melt, then obtaining a very thin product with sufficient property.
In the invention, the molten metal is fed at a higher velocity at a product gate into the mold cavity to increase the fluidity of the semi-molten melt, then further improving the quality of the very thin molded product.
In the invention, solid fraction Fs in the semi-molten melt to be fed is defined as being in lower levels in relation to grain size D (xcexcm) of the solid phase in the melt so as to increase in fluidity in feeding the melt into the narrow mold cavity, then, obtaining thin, sound products.
In the invention, an overflow gate is provided in the mold at a opposite side of the cavity to the product gate and the thickness of an overflow gate portion of the thin molded product corresponding to the overflow gate is set to be lower than the thickness of the product gate portion corresponding to the product, then, achieving sufficient degassing of the cavity to the overflow groove formed continuously past the overflow gate. This thereby improves the quality of the thin molded article as a whole.